This invention pertains generally to the field of power regulation and more particularly to a power regulator having discrete states of regulation.
As electronics become more sophisticated, the demands on power regulators have increased. For example, modern microprocessors need power supplies providing lower voltages at higher currents. Whereas in the past, a microprocessor might need a regulated power supply providing a maximum of 15 amps at 3.2 volts, a modern microprocessor may require a regulated power supply of 100 amps at 1.8 volts. Such a microprocessor would draw little current if in a dormant mode but would demand up to 100 amps of current during moments of heavy load. Given the high speed of these devices, the transition between low and high power demand may occur vary rapidly.
Linear regulators have been used to provide regulated power to microprocessors. A typical linear regulator is illustrated in FIG. 1. A differential amplifier, U1, compares the output voltage, V_out, to a reference voltage, V_ref, and adjusts the current drive to the base of the pass transistor, Q1, to make V_out track V_ref as the load current and input voltage, V_in, vary. If such a linear power regulator is used to regulate the power supply for a modern microprocessor, its slew rate will not accommodate the rapid transition between low and high current demands. Moreover, linear regulators are inefficient and tend to have high maintenance needs.
Avoiding the inefficiencies of a linear regulator, U.S. Pat. No. 5,969,514 discloses, as illustrated in FIG. 2, a plurality of power field effect transistors (FETs) M1-M8 arranged in parallel between an input voltage, Vin, and a load 13. A control circuit 20 maintains the FETs M1-M8 either in cutoff (OFF) or in saturation mode (ON). The control circuit 20 switches M1-M8 ON or OFF according to a digital feedback signal proportional to a voltage, VOUT, on the load 13 as measured by an analog-to-digital converter 5. The control circuit 20 compares the digital feedback signal to a reference signal, VREF, and switches ON or OFF a varying number of the FETs M1-M8. During moments of little power demand by the load 13, only a relatively small number of the FETs are ON. However, during moments of maximum power demand, all the FETs are ON. Because the saturation resistance of identically produced FETs tends to be quite similar, the FETs M1-M8 may be modeled as eight resistances R arranged in parallel, where R is the saturation resistance. If only one FET is ON, the resistance between the input and output is R. If all the FETs M1-M8 are ON, the resistance is R/8. In general, if N of the FETs are ON, the resistance is R/N. In this manner, the control circuit 20 determines a resistance between the input and output, where the resistance takes on discrete values as given by the number of conducting FETs.
Although the power supply of FIG. 2 efficiently keeps the FETs either in cutoff or saturation mode, it suffers from a number of disadvantages. For example, consider the case of an input voltage, Vin, having both positive and negative (AC) values. Because the source of power FETs is typically coupled to both the input voltage and the substrate, the FET, when ON, acts as a diode whose cathode is the drain and anode is the source. The resulting effective diode from the drain to the source will conduct, even though the FET is OFF, if the source is sufficiently lower in voltage than the drain. Such a scenario is possible in the case of an alternating voltage input, preventing power FETs from being bi-directional switches and preventing the power supply of FIG. 2 from using an AC input voltage.
Thus, there is a need in the art for improved power regulators that maintain high efficiencies over a broad range of load conditions with AC voltage inputs.
The invention provides in one aspect a power regulator having a plurality of bi-directional switches connected in parallel between an input and an output. A controller regulates an output voltage by switching ON a subset of the plurality of bi-directional switches while maintaining the remainder of the plurality OFF. The controller switches ON or OFF the subset in response to comparing the output voltage and/or an output current to a threshold level. In addition, the controller may also provide synchronous rectification at the output by switching ON the subset only when an input voltage exceeds the output voltage.
Other aspects and advantages of the present invention are disclosed by the following description and figures.